1. Field of the Invention
The invention described herein relates to communications systems, and in particular relates to frame synchronization.
2. Description of Related Art
Free-space optical communications benefit from the advances in photonics that have enabled fiber-based optical systems to achieve a prominent role in wired backbone networks. Despite this success, however, physical layer and link layer protocols developed for fiber optics systems may not translate successfully to free-space optical applications.
This is seen in the frame synchronization process for packet communications over free-space optical links. Generally, a transmitter sends binary data to a receiver. The binary data is organized into frames of N bits. The receiver sees a continuous stream of bits and must determine the frame structure of the received data. In particular, the receiver needs to determine where each frame starts. This is required for a number of processes, including but not limited to forward error correction. The process of determining the starting point of frames is fundamental to frame synchronization.
One mechanism by which frame synchronization may be achieved involves the insertion of a known preamble into each frame. A transmitter may, for example, add a known preamble that is L bits long at the beginning of each frame. If the entire frame is N bits long, then the preamble will be followed by N−L data bits. The receiver must therefore find the preamble in each frame in order to determine the starting point of the frame. Finding the preamble is therefore required in order to attain frame synchronization.
This poses a processing problem for packet communications over a noisy channel as is the case for wireless radio and free space optical systems. The receiver must identify a bit pattern that matches the known preamble despite the fact that the received signal contains random user data and each bit signal may be corrupted by noise. Note that the communications receiver must compare the received noisy signal to a threshold in order to determine whether a 0 is present or a 1 is present. Such a determination may only be probabilistic in a noisy environment. The possibly noise-corrupted random user data further complicates the preamble identification because it might match or partially match parts of the preamble.
What is needed, therefore, is a system and method by which preamble identification may take place with a relatively high probability of accuracy by minimizing errors caused by noise or random user data.